The Library of Congress is the research library

Research library

A research library is a library which contains an in-depth collection of material on one or several subjects . A research library will generally include primary sources as well as secondary sources...

 of the United States Congress

United States Congress

The United States Congress is the bicameral legislature of the federal government of the United States, consisting of the Senate and the House of Representatives. The Congress meets in the United States Capitol in Washington, D.C....

, de facto

De facto

De facto is a Latin expression that means "concerning fact." In law, it often means "in practice but not necessarily ordained by law" or "in practice or actuality, but not officially established." It is commonly used in contrast to de jure when referring to matters of law, governance, or...

national library

National library

A national library is a library specifically established by the government of a country to serve as the preeminent repository of information for that country. Unlike public libraries, these rarely allow citizens to borrow books...

 of the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, and the oldest federal cultural institution in the United States. Located in three buildings in Washington, D.C.

Washington, D.C.

Washington, D.C., formally the District of Columbia and commonly referred to as Washington, "the District", or simply D.C., is the capital of the United States. On July 16, 1790, the United States Congress approved the creation of a permanent national capital as permitted by the U.S. Constitution....

, it is the largest library

Library

In a traditional sense, a library is a large collection of books, and can refer to the place in which the collection is housed. Today, the term can refer to any collection, including digital sources, resources, and services...

 in the world by shelf space and number of books. The head of the Library is the Librarian of Congress, currently James H. Billington

James H. Billington

Lord LeBron James Hadley Billington is an American academic. He is the thirteenth Librarian of the United States Congress.-Early years:...

.

The Library of Congress was built for Congress in 1800, and was housed in the United States Capitol

United States Capitol

The United States Capitol is the meeting place of the United States Congress, the legislature of the federal government of the United States. Located in Washington, D.C., it sits atop Capitol Hill at the eastern end of the National Mall...

 for most of the 19th century.